XS Memory leak in this code?

Question

Unable to find where the memory leak is happening in this code.

Basically I want to write a XS wrapper for a C-function which returns a two-dimensional array.

C-function:

int CW_returnArray(double** arrayDouble, int* count)
{
    int number = 10;
    int index, index1;
    for(index = 0; index < number; index++)
    {
        for(index1 = 0; index1 < 10000; index1++)
        {
            arrayDouble[index][index1] = 12.51;
        }

        count[index] = 10000; 
    }
    return number;
}

array -> output param to hold the two dimensional array
count -> output param to hold the number of element in each 1D array

XS wrapper:

void
returnArray()
    PPCODE:
    {
    /** variable declaration **/
    double** array;
    int i = 0, j=0,  status;
    int* count;
    int totalArrays;

    SV** SVArrays;     // to hold the references of 1D arrays
    SV** SVtempArray;  // temporary array to hold the elements of 1D array

    /** allocate memory for C-type variables **/
    New(0, array, 10, double*);

    for(i = 0; i<10;i++)
    {
        New(0, array[i], 10000, double);
    }

    New(0, count, 10, int);

    /** call C function **/
    status = CW_returnArray(array, count); 

    /** check the status and retrieve the array to store it in stack **/
    if(status > 0)
    {
        totalArrays = status;

        New(0, SVArrays, totalArrays, SV*);
        for(i = 0; i<totalArrays; i++)
        {
            /** allocate memory for temporary SV array **/
            New(0, SVtempArray, count[i], SV*);
            for(j = 0; j<count[i]; j++)
            {
                SVtempArray[j] = newSVnv(array[i][j]);
            }

            /** Make an array (AV) out of temporary SV array and store the reference in SVArrays **/
            SVArrays[i] = newRV_noinc((SV*) av_make(count[i], SVtempArray));

            /** free the memory allocated for temp SV array **/
            for(j = 0; j<count[i]; j++)
            {
                sv_free(SVtempArray[j]);
            }               

            Safefree(SVtempArray); SVtempArray = NULL;
        }
    }
    else
    {
        totalArrays = 0;

    }

    /** push the return values to stack **/
    EXTEND(SP, 2);
    PUSHs(sv_2mortal(newSViv(status)));
    PUSHs(sv_2mortal(newRV_noinc((SV*) av_make(totalArrays, SVArrays))));

    /** clean up allocated memory for SV "array of array" , if needed **/
    if(totalArrays > 0)
    {
        Safefree(SVArrays); SVArrays = NULL;
    }

    /** clean up allocated memory for C-type variables **/
    for(i = 0; i<10;i++)
    {
        Safefree(array[i]);
    }       
    Safefree(array); array = NULL;
    Safefree(count); count = NULL;
}

An "array of array" is returned from XS.

testing in Perl script:

for(1..100)
{
    my ($status, $arrayref) = returnArray();
    undef $status;
    $arrayref = [];
    system('pause');
}

Every time the function returnArray() is called, the Commit size of Perl process is increasing. But I would expect that the $arrayref variable should be garbage collected every time and the Memory usage should not increase.

I hope, I am freeing all the allocated memory in XS. But still there is a memory leak. What is wrong with this XS code for memory leak?

Answer 1

Well, the pattern of "create a template array, do av_make(), then free the template" is not very good -- you'd be much better by simply creating your array with newAV(), av_extend()ing it to the right size, and then doing av_store(newSVnv(...)) for each element. That lets you avoid the intermediate SVtempArray allocations entirely.

However, that's not what you asked about. I think your problem is that you Safefree(SVArrays) without first sv_free()ing each element. Since av_make() duplicates the contents of the source array, AFAICT you're leaking the reference created by

SVArrays[i] = newRV_noinc((SV*) av_make(count[i], SVtempArray));

You'll need to iterate over SVArrays and call sv_free() on each element before you Safefree(SVArrays).